Previous work in our laboratory has indicated translocation of prolactin (PRL) to the nucleus upon receptor internalization. To identify potential PRL-binding proteins that may facilitate this translocation, interactive screening was performed. These screens identified a member of the immunophilin family, cyclophilin B (CypB), as interacting with PRL. The cyclophilins are a family of peptidyl-prolyl isomerases (PPI) that serve as protein chaperones and mediate the immunosuppressive effects of cyclosporin A (CsA). Our data demonstrate that the interaction of CypB with PRL and another somatolactogenic hormone, growth hormone (GH), is enhanced in the presence of CsA. This interaction results in a ten-fold increase in PRL-driven proliferation and a significant increase in the retrotranslocation of PRL into the nucleus, an event that is abrogated by the removal of a putative nuclear localization signal sequence in the amino-terminal of CypB. Therefore, it is hypothesized that CypB-PRL interaction facilitates PRL internalization and assists in its translocation to the nucleus. This hypothesis will be tested by two specific aims using PRL-responsive lymphoid and breast cancer cell lines. First, the interaction, composition, and function of the CypB/PRL/PRLr complex will be determined by biochemical and mutagenic approaches. Second, the mechanisms and functional consequences of CypB/PRL internalization will be examined through pulse-chase studies using biochemical and electron microscopic approaches in conjunction with CypB and PRLr mutants. These studies will provide significant new insight into the physiological role of the cyclophilins and further define the mechanisms through which PR/PRLr internalization are mediated in PRL-responsive tissues.